In general, in a high electron mobility transistor (HEMT), that is, a compound semiconductor device, one or more layers included within the device have very different lattice constants from those of other materials within the device. Due to such a lattice mismatch, the structure of a material forming a channel layer is deformed. In the HEMT, stress distortion caused by such a lattice mismatch improves electron mobility in the channel layer, thereby improving the operation speed of the device.
The HEMT has a difficulty in substrate growth, but has an increase in the density of charges transferred to the channel layer, and a high electron mobility. In other words, the HEMT has a higher power and an improved noise characteristic. Accordingly, the HEMT can be operated in a high frequency. Further, the HEMT is more excellent in an electron speed characteristic than an electronic device using silicon, and thus is widely applied to microwave or millimeter wave band devices. Especially, since the HEMT has advantages such as a low super-high frequency noise characteristic, the HEMT is an important device used to develop millimeter-wave band circuits and components with high-performance for wireless communications.
Meanwhile, in a high-speed device, a gate length has to be decreased to achieve a high modulation operation, and further, it is required to improve a noise characteristic by reducing a gate resistance. Thus, a T-gate or mushroom-gate having a wide cross-sectional area is essentially used.
The T-gate or mushroom-gate is generally formed through an electron beam lithography method or a photolithography method. However, since in the photolithography method, the resolution was insufficient to form a fine line width of a gate electrode, the electron beam lithography method has been conventionally used to form a T-gate electrode. In the electron beam lithography method, a double-layered or triple-layered photosensitive film is generally used.
FIG. 1 is a cross-sectional view illustrating the configuration of a transistor having a conventional a T-gate electrode structure.
As shown in FIG. 1, on a substrate 103, a source electrode 109a and a drain electrode 109b are in ohmic contact with each other, and an insulating film 111 and a gate electrode 113 are formed.
However, in the transistor having a conventional T-gate electrode structure, the width of a gate length may be increased due to wet etching, and a high frequency characteristic may be deteriorated due to an increase of gate-source and gate-drain capacitance.
Further, in the transistor having the conventional T-gate electrode structure, since wet etching is performed by using an etch stopping layer 105, it is required to accurately adjust an etching rate. Since an undercut may be formed due to an etching characteristic in which the wet-etching is performed in lateral directions not only in a depth direction, source resistance may increase and a gate length may be changed. This may have an influence on the improvement in performance of a device.